Will algae raise the pH level in a tank?

Actually it can have an impact on you pH because it is a plant that is releasing oxygen in the day, and CO2 at night. You pH will generally be lower in the morning and higher in the evening because of the photosynthesis of the algae. This is true for all plants.

Not significantly. Any biological action in the tank will have an effect on the pH, but not enough to make any difference to the fish.

If your pH is going up over time it's probably due to having some lime based rock or sea shells in the water.

pH is also not as important as many people think and most fish are able to surive a pretty wide range.

Ian

No, all algae will do is compete for any live plants in the tank for nutrients and depending on the type, it'll make your water a green color. There is nothing wrong with it. I keep a patch of string algae growing on a flat rock in my aquarium as I like the look of it. It will not raise the pH of your tank though.

No, real algae is a bacteria, not a plant. It won't really bother your pH. It might mess with your oxygen levels though.